Fine wires, also known as bonding wires, are commonly used in integrated circuit applications for connecting an electrode on a semiconductor element to an external terminal. For example, suitable fine wires may have a wire diameter of approximately 0.020 inches (0.508 micrometers) to 0.080 inches (2.032 micrometers). Bonding wires may be bonded using a thermal compressive bonding technique. For example, a leading edge of a bonding wire may be heated to form a ball by surface tension. The ball may then be compressively bonded on an electrode of a semiconductor element. Thereafter, the bonding wire can be directly bonded to an external lead by, for example, a wedge bond.
Suitable bonding wires may be made of lead (Pb) or lead alloys. Lead bonding wires have a low material cost and excellent electrical conductivity. However, oxidation of the lead bonding wire typically reduces bonding strength.
Fine wires made from metal or metal alloys are manufactured to tight tolerances to ensure uniformity in the composition and material properties of the fine wire. Metal or metal alloy wires are often prone to defects such discontinuities, material segregation, or material gradients. One problem that can occur during the manufacture of wires is formation of discontinuities within the material that forms the wire. Examples of discontinuities include voids, blisters, cracks, and changes in porosity throughout a material, as well as solid inclusions or impurities. Discontinuities in a wire can lead to wire breakage during further processing, transportation or handling of the wire. If the wire is to be used as a solder, solid inclusions such as impurities can result in particles of the impurities being deposited onto the solder.
Open air casting using vertical book molds may lead to voids being formed in the alloy due to solidification in the molds. Alloy wires that contain voids or inclusions often result in sparking or spitting issues when the wire is later used, for example when used to solder electrodes of a semiconductor element. Alloy wires that contain voids or inclusions have a tendency to break at the location of the void or inclusion. Frequent wire breakage leads to lower production yields.
Additionally, segregation within the metal alloy may lead to localized changes in the concentration of individual components of the cast metal alloy and may occur along the length of the wire, radially or in both directions. Localized differences in concentration can lead to weakening of the fine wire. Segregation or non-uniform concentrations of individual components of the cast metal alloy may also result in material property gradients. Because individual components may have different material properties, such as melt temperatures or the coefficient of thermal expansion, identical process parameters may lead to inconsistent effects on various parts of a fine wire, making suitable processing parameters difficult to predict.
There is thus a need for a method of forming an alloy wire that is substantially free of voids, inclusions, and segregation.